1. Field of the Invention
The present invention relates to an optical waveguide forming epoxy resin composition and an optical waveguide forming curable film to be used as a material for a cladding layer of an optical waveguide of an optical waveguide apparatus widely used for optical communications, optical information processing and other general optics. The invention also relates to a light transmission flexible printed board and a production method for the flexible printed board.
2. Description of the Related Art
Optical waveguide cladding materials for light transmission flexible printed boards are required to have higher curing sensitivity (excellent patternability), higher flexibility and lower refractive index. In designing a material satisfying such requirements, an aliphatic resin is typically selected for the lower refractive index, and a multi-functional aliphatic epoxy resin and a long-chain bi-functional aliphatic epoxy resin, for example, are blended as required to impart the material with excellent patternability (higher sensitivity) and higher flexibility. For a cladding material particularly required to have higher flexibility, therefore, the amount of the long-chain bi-functional aliphatic epoxy resin is inevitably increased, so that a cladding layer formed by curing the cladding material tends to have a lower glass transition temperature Tg (see, for example, JP-A-2011-27903 and JP-A-2010-230944).
In a roll-to-roll process for mass production, on the other hand, a dry film technique is generally employed, in which an uncured film is used in the form of a dry film material. In order to adapt the dry film material for the roll-to-roll process, the uncured material is required to have lower tackiness and higher flexibility. In development of the material, however, these requirements reduce the material design flexibility. In production of the dry film, it is necessary to provide laminate substrates on opposite surfaces of the dry film, resulting in problems associated with resource saving and cost saving. In the material development, it is also important to adapt the material for a wet process in which a liquid material is applied onto a substrate for formation of a cladding layer and a core layer (JP-A-2010-230944).
In the wet process, a surface smoothing agent (leveling agent) is added to the material or a solvent having a higher boiling point is used to increase the surface smoothness of a coating film in a coating film forming and solvent drying step.
However, the addition of the leveling agent reduces the surface tension of the surface of the coating film, resulting in repellency of a coating liquid in the subsequent coating film forming step. This often adversely influences a waveguide loss.
Where the higher boiling point solvent is used instead of the leveling agent, on the other hand, the following problem will be encountered. Where a cladding layer material imparted with the flexibility and hence having a reduced glass transition temperature Tg is used for formation of an under-cladding layer, the higher boiling point solvent in a core layer forming material is dried at a heating temperature higher than the glass transition temperature Tg of the under-cladding layer forming material in a core layer forming and drying step in which a core layer is formed on the under-cladding layer formed from the cladding layer material. Therefore, a resin component of the core layer forming material having a higher refractive index is liable to infiltrate into the under-cladding layer having a lower refractive index. Therefore, light propagating through the core layer is liable to leak into the under-cladding layer, thereby increasing the optical waveguide loss.
Where an optical waveguide is produced in an application which requires the positional accuracy of the optical waveguide (e.g., in production of an opto-electric hybrid board), it is necessary to form three layers including an under-cladding layer, a core layer and an over-cladding layer by a photolithography process (including a coating film forming step, a solvent drying step, an alignment exposing step, a post heating step, a developing step and a drying step for each of the three layers). This increases the production costs. A technical solution to this problem is highly difficult, but omission of the solvent drying step is a relatively easy approach for reduction in the number of the process steps. Therefore, it is essential to develop a material which permits the omission of the solvent drying step.
Besides the material-based approach, an approach associated with the production process is also contemplated. In this approach, a mask coating process or a dispenser drawing process which can shape a coating film in the coating film forming step without the need for the alignment exposing step, the developing step and the drying step for the reduction in the number of the process steps is required as a coating film forming process for forming the cladding layer.
This coating film forming process, if using a coating varnish containing a solvent, essentially needs the solvent drying step (pre-baking step), making it impossible to maintain the configuration and the dimensions of the cladding layer because of reduction in resin viscosity during heating. The essential need for the solvent drying step correspondingly adds process costs. Therefore, a solvent-free coating varnish having a higher viscosity at an ordinary temperature is demanded.
However, preparation of such a coating varnish on an industrial scale encounters the following problems. If a solid resin component is present in the solvent-free system, it is difficult to mix the solid resin component with other resin component. In the preparation of the coating varnish, therefore, an excessive long heating and stirring period is required. Further, it is difficult to provide a coating varnish having an intended formulation with the solid resin component left undissolved. In addition, when the coating varnish is exposed to a lower temperature environment, the solid component is liable to be deposited to cause haze.
With such technical background, the coating varnish preferably contains only a liquid resin as a resin component in order to simplify the coating varnish preparation step and reduce cycle time (process time). There is an eager demand for a solvent-free coating varnish as a cladding layer forming material which satisfies fundamental optical waveguide property requirements (e.g., a higher glass transition temperature Tg and higher flexibility) and contains only the liquid resin as the resin component.